If you see something advertised as an injury-prevention tool and Arsenal one of the teams using it, it is but normal to be suspicious of the product. But quips aside, one of the ways that football is being dragged, kicking and screaming, into the modern age is via performance-monitoring technology.

To follow up on the El País story published about FC Barcelona players wearing monitoring vests as part of an overall training and injury prevention program, it seemed worth digging a bit deeper into the STATSports product, to know what it does and try to explain just a bit about why it can work.

An athlete gets a knock during a match and we all see the trainers come out with the magic water, an absurd practice that is similar to your Mom kissing an owie to make it all better. Players do the same warm-ups and routines in a game that generates loads of cash yet sometimes relies on voodoo when it comes to player maintenance. These monitoring vests from Northern Ireland-based STATSports are, for athlete-tech nerd types who also love football, too cool for words.

As a cyclist, I ride with a power meter that gives me all pertinent training data at a glance: Heart rate, training intensity, power output, speed, altitude… It can even tell me how to get home if I am too tired to find my way. The value of this data is that it allows an athlete to devise a training plan and stick to it, an approach based on scientific parameters. If I, a weekend warrior old fart can have this, football should have more than some grizzled veteran saying, “Adriano looks tired. We should rest him tomorrow.”

The STATSports vest (yes, the jogbra-looking thing) is a step in that direction. Using GPS tracking, data can be gleaned such as distance, speed, heart rate variability, accelerations, decelerations, impact, dynamic stress load, and other goodies such as metabolic power. Increasingly, people are realizing that injuries are not bad luck, but fatigue-based. A tired muscle loses hydration and elasticity. Elongate or compress it the wrong way one time too many and it ruptures or tears. Then Bayern stomp you 7-0 on aggregate as your star player hobbles around. If you want to run training at match intensity, you know when a player is dogging it.

Of the training statistics that the STATSports vest can track, let’s have a look at each and how it might apply to a Barça player:

Distance: Kilometers covered. How far. If coaches know that a player has a good 40 km per match in him and he logs 30 km in the first half, informed decisions can be made in terms of substitution.

Speed: How fast. If Messi runs 30 km/h in the first half but can only generate 20 km/h in the second, something has to happen. But because no coach alive will sub him, something else has to happen. With real-time data, a performance dropoff that might lead to an injury or a chronic fatigue crisis can be spotted, prompting the sideline command, “Walk, Leo, walk!”

Heart rate variability: Every athlete has a resting heart rate. There are also heart rates at levels from warming up to full sprint to those lull periods in every match. An athlete who is fit and feeling good will have a heart rate that returns to baseline in X seconds after a hard effort. If it takes Y seconds, that is usually an excellent indicator of fatigue. A tired athlete will also have a hard time getting the heart rate into a working zone as well as a hard time with the heart rate returning to baseline. Heart rate is one of the more significant parameters in performance monitoring.

Accelerations: How many and how hard. Just as a simple accelerometer can tell you how fast your car accelerates, the GPS data can give coaching staff the necessary information to decide that Alba shouldn’t be sprinting that many more times, as a smart training staff will already know how many hard efforts it takes to tire him. Notice that Messi walks or trots sometimes instead of standing still. The acceleration forces on an already-moving muscle are significantly lower than a stationary one. So walking Messi will have a decreased likelihood of injury than taxicab Messi. It isn’t the movement as much as the force necessary to create the movement.

Decelerations: Those are more violent and stressful than accelerations for the same reason it is often more difficult to walk or run downhill than uphill. Not only are the muscles that are used different, but deceleration stresses make those muscles contract in a violent way compared to the elongation stresses of acceleration. Deceleration (an unfortunate landing, in this instance) changed Victor Valdes’ career. Deceleration is also an advantage for certain players. Neymar and Messi stop as quickly as they start, an extraordinary gift that requires flexibility of muscles and tendons as well as extraordinary strength. The reason Neymar can stop while his defender keeps going isn't just because the player knows when he is going to stop. Deceleration training is being increasingly used as a way to prevent ACL and hamstring tears, for example. How you slow down is as important as how you accelerate.

Impact: How hard. But more than that, it’s force applied as a different subset of power. Part of the testing of an athlete should be what the joints can take. Box jumps, or leaping over those hurdles that you see in training, subject the legs to impact stresses. The higher you jump, the greater the impact when landing. A sprinting athlete produces forces many times their body weight. Impact forces beyond certain parameters can increase the possibility of stress-related injury. Though we always think of impact as how hard Iniesta just got kicked, how hard he hits the ground while running will be more significant to a training staff. Getting kicked is a different type of impact than running or jumping. These latter two are much more interesting as a predictor for a training staff.

Dynamic stress load: This relates to acceleration and deceleration. When an injury occurs there is usually an abnormal increase in dynamic stress load at the point of the injury. That Valdes injury looked like a routine play, but seeing the stress loads generated by his landing across the knee joint would be interesting. "He landed funny" is a layman's way of saying that something wasn't right. And as he came down, it's easy to speculate that the misapplied force had a lot to do with his injury. Athletes not only have to learn how to run in a way specific to their bodies, but how to slow down and stop.

Metabolic power: How much force is required to initiate a movement. Watts have become the king of cycling training. Everyone is using watts to define performance, whether maximum sustainable, peaks or average over a period of time. When you initiate a sprint it isn’t just force that is generated, but watts that are required. To break it down to a more specific example, my cycling discipline is sprinting. When we work on standing starts, data is gathered in increments: Peak power, 5-second power, 10-second power, 30-second power and 1-minute power. Power, as defined by work load, will diminish over the course of an effort. So while a peak might be north of 2,000 watts, the 1-minute number would be perhaps half that.

As Messi sprints, the metabolic power that he generates will be prodigious. Though power numbers on Messi aren’t available, in the 2009 world championships Usain Bolt had a peak wattage of 2,619 watts, which came within the first second of his effort, a staggering number. This makes sense as standing starts always generate the highest wattage spikes. It’s another reason that walking or trotting Messi is better than taxicab Messiless metabolic force is generated to produce a run, which means Messi can be… well… Messi for longer and stronger.

Performance data has been in use for a number of years, but this is the first year that Barça ha embraced the technology. The El País story says that the vests “warn about injury risks by quantifying muscular fatigue values.” It is actually a lot more complicated than that, because the vests cannot actually quantify muscular fatigue … yet. But they can measure the above factors and, as the training staff compares it to each player’s baseline, more accurately predict the presence of fatigue stressors. With half as many injuries this season and only one hamstring knock (compared to nine last season), the proof appears to be in the pudding.

Disclosure: This article isn't sponsored by STATSports or any of their affiliates.

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